As determined by intracellular recording of the action potential, the electrical properties of both chick and rat heart change dramatically during embryonic and fetal development. The primary objectives of the proposed research are (1) to describe in detail the time- and voltage-dependent membrane conductance mechanisms which underlie the action potential and its spontaneous generation in cultured aggregates of embryonic heart cells: and (2) to observe how such properties change during development. The two-microelectrode voltage clamp technique will be employed to determine the magnitude, kinetics and voltage dependence of trans-membrane currents, information which cannot be obtained from intracellular recordings. The specific ions which carry these currents will be identified by measurements of reversal potentials and peak current magnitudes under conditions employing elevated or reduced concentrations of Na ion, Ca ion ion, K ions or Cl negative ion. Finally, cycloheximide, an inhibitor of protein synthesis, will be used to determine whether synthesis of new proteins is required for differentiation of these electrical properties.